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"Class A" vs "Class B" Signal Booster/ BDA (Part 90, Industrial)

What are the differences between Class A and Class B signal boosters or bidirectional amplifiers (BDA) per their Industrial Grade Part 90 classification? Which one is better, and why?

FCC Part 90 Signal Booster Classifications:

  • Class A signal booster. A signal booster designed to retransmit signals on one or more specific channels. A signal booster is deemed to be a Class A signal booster if none of its passbands exceed 75 kHz.
  • Class B signal booster. A signal booster designed to retransmit any signals within a wide frequency band. A signal booster is deemed to be a Class B signal booster if it has a passband that exceeds 75 kHz.

Making the Choice Between Class A and Class B Cellular Amplifiers.

When choosing between Class A and Class B devices, it is important to understand that your Authority Having Jurisdiction (AHJ) does ultimately have the final choice. If the relevant municipality has adopted its own ordinance or code, the local AHJ should be able to give you a copy of their documentation or direct you to their website. In some instances, your local AHJ will not specify whether Class A or Class B is required but will leave this decision up to individual system integrators to ensure that the relevant performance criteria are met at all times.


Class A devices tend to be more expensive than similar Class B devices because Class A devices offer greater levels of RF control and have higher levels of built-in intelligence than their Class B counterparts. The difference in price between otherwise similar Class A devices and Class B devices will understandably vary between vendors. However, in general, it is fair to say that a Class A device will cost approximately 30% more than a similar Class B device with roughly the same RF power output levels.


In general, Class B devices may have easier set-up instructions since they have fewer options to choose between. By way of example, a Class B device may not allow a user to change the filter bandwidth or may limit individual set-up options to redefining a pass window by specifying stop and start frequencies. In contrast, almost all Class A devices allow for the individual specification of desired frequencies, while some Class A devices will allow bandwidth options to be specified, along with squelch and channel gain on an individual basis.


When looking at the reliability of Class And Class B devices, the first point to note is that both classes have very similar MTBF (Mean Time Between Failure) ratings. Where performance differences do exist is in the ability of devices in each class to maintain system performance that is reliable. Class A devices have the ability to filter channels that are considered undesirable, ensuring that such channels do not take away amplifier power from important channels. In addition, Class A devices can maintain output levels that are stable while absorbing minimal fluctuations due to their gain circuits, which are considered adaptive or dynamic.

Class B devices, on the other hand, can undergo relatively major fluctuations in power output due to the RF wideband environment channel loading. Such fluctuations can have an adverse impact on coverage within a building, which can sometimes drastically affect radio communications relied upon by first and emergency responders.

Site Noise.

Class A devices have the ability to stay silent until such time as radio keys are activated inside the building, due to their amplifier circuits, which are considered independent. This means that the only channels transmitted to the donor site are active channels. In contrast, Class B devices are always switched on and always causing noise amplification, even in situations where no or very little radio traffic is occurring inside the building.

Site noise is an issue of growing importance, primarily due to code requirements and the implementation of BDAs. With Class B devices continuing to add noise generation to the environment, donor sites are ultimately affected. For this reason and the sensitivity of donor sites themselves have led many AHJs to adopt a new position on Class B broadband devices.

As a general rule, a Class A device that has been set up with a well-placed squelch circuit can be expected to generate approximately 30 dB less noise than a similar Class B device. Even better, technologically advanced Class A devices, for instance, the CriticalPoint Class A by Comba, will generate no noise at all and appear completely silent until the appearance of legitimate radio traffic.

Near-Far Affect.

In the context of DAS systems for Public Safety, the near-far effect refers to the situation where one portable is placed nearby to an inside antenna and another portable is simultaneously placed at a distance, referred to as X, which can be thought of as the midpoint between both antennas. When the closest radio is keyed, the Class B device is forced to reduce its gain in order to prevent a destructive overload situation or saturation. When this occurs, the radio situated at the position X may not be able to properly and reliably communicate with the donor site due to insufficient levels of gain. This situation has caused changes to various codes, with many municipalities now requiring radios to be tested in the event of the implementation of a Class B system. The near-far effect can be avoided with proper design and by using an antenna that is of sufficient density, which can be measured by way of this two-radio test.

By contrast, a set-up involving Class A devices will not incur problems related to the near-far condition. This is because each channel has independent gain available to it, which means that the positioning of near and far radios will have no effect on each other when it comes to either radio's ability to connect with the donor site.


When considering scalability issues, Class B devices manage to come out on top of their Class B alternatives. It remains true that Class A devices can accommodate the majority of channel counts that are macro network related by way of programming, but in the event that a further frequency is introduced at some future point (and such frequency does not fall within the existing pass windows), then it will be necessary for manual changes to be made via a remote connection or by a physical site visit. Class B setups, however, allow for any frequencies appearing within the general passband of operation to be automatically amplified. It is this scalability issue that is one of two major reasons why many AHJs continue to choose Class B devices over their Class A alternatives. The second major reason is TDI, also known as a delay.


TDI, or Time Delay Interference, occurs when the donor site's direct signal is able to penetrate through the walls of the building and comes against the delayed signal amplitude distributed by way of the BDA DAS. Class B devices are a subset of broadband devices and feature a wide filter that can introduce a delay of less than 10µsec. On the other hand, a Class A device is a type of narrowband device and is designed only to pass frequencies that are discrete, due to its very sharp and fine filtering. As a result, the delay introduced by Class A devices tends to range between 15µsec and 100µsec, depending upon filter options and the equipment manufacturer.

Municipalities that choose to use radio communications that are analog, in general, are not affected by delay. On the other hand, Phase II and Phase I P25 digital devices may be more likely to be influenced by issues relating to delay. A radio's tolerance of delay will naturally vary by manufacturer and technology levels. However, as a general idea, coverage dead spots can be minimized by including a maximum of 15 µsec in Phase II P25 devices and a maximum of 32 µsec in Phase I P25 devices, unless, of course, you are able to maintain signal dominance. That being said, most of today's Class A devices do include an option for users to adjust delay settings in order to optimize their system performance to suit the relevant RF environment.

Another way to effectively mitigate TDI is in the creation of signal dominance. The majority of national and local codes include the requirement that communication by radio when at a DAQ (Delivered Audio Quality) level of 3.0, meets coverage of a minimum of 95%. Depending on the technology utilized, this roughly translates to between 16 and 18-dB SNR. Issues related to TDI can be eliminated altogether by creating a system within a building in which the macro site signal is dominated by the indoor signal by a minimum of 16 to 18 dB. The construction of the majority of today's buildings does conform to Lead Certification levels, which makes the achievement of signal dominance within a building much easier.


Differences on the surface level between Class A and Class B devices - in other words, broadband versus narrowband can be relatively easy to understand. However, when looking at these differences in a real-world application, the situation becomes much more difficult. Generally, the best plan our installation estimate team proposes is to begin with your Authority Having Jurisdiction (AHJ) to see if a local Code stipulates either Class A or Class B. In the event that it doesn't, hopefully, what you have learned in this article will assist you in making your decision.

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